Vaccines comprising a combination of protective antigens derived from different pathogenic organisms have multiple obvious benefits, both for the recipient—and manufacturer of the vaccine. In particular, combination- or multivalent vaccines offer increased ease of administration and greater comfort and convenience to the patient by reducing the number of injections required and possibly the number of attendances. They are also more economical to manufacture and administer because of savings on processing of combined bulk material, containers, packaging, distribution and injection equipment.
In the field of human health combination vaccines are often used in the context of infant vaccination. Combination vaccines such as DTP (diphtheria, tetanus and pertussis), with or without inactivated poliomyelitis and MMR (measles, mumps and rubella) have been in use for many years and new antigens have been added to this combination during the last years.
Also in the field of animal health combination vaccines are commonly used. In particular vaccines of poultry, swine, ruminants and companion animals are, more often than not, based on a combination of multiple antigens. Examples of such vaccines are combination vaccines against canine distemper, hepatitis, parainfluenza type 2, parvovirus, leptospira and rabies virus for dogs, rotavirus, coronovirus and E. coli for cattle, Newcastle disease virus, infectious bronchitis, infectious bursal disease, swollen head syndrome and egg drop syndrome for poultry.
A vaccine batch may be released for sale only after a license or marketing authorization has been issued. In addition, each subsequent batch of such an authorized vaccine batch has to be formally released in compliance with the rules of a state or number of states concerned. This release may be permitted on the authority of the manufacturer after satisfactory completion of the prescribed batch testing. Therefore, to guarantee that each and every batch of vaccine will have its intended effect, a manufacturing process of consistent quality has to be in place, and the application of a potency test is an essential element of such a process.
Presently, different test methods, such as assays of physiochemical properties, antigenicity, immunogenicity, infectivity and protection against infection or disease, are used to measure vaccine potency. Their application depends on the nature of the vaccine antigens and the purpose of the test. In live vaccines, potency can be based on the number of organisms present in the vaccine (titre). In the case of inactivated vaccines, the potency is often determined by measuring the immune response in the target animal species or in another species, e.g. mice or rats. Alternatively, the potency of an inactivated vaccine can be based on its antigenicity by measuring the quantity of the antigen present (antigen mass), using immuno assays that employ specific antibodies, such as an ELISA (enzyme-linked immunosorbent assay).